Satellites carrying radar altimeters record the surface topography along the satellite’s ground track. They precisely measure a satellite’s height above water, land or ice by timing the interval between the transmission and reception of very short radar pulses. This is the only technology that can measure, systematically and globally, changes in the height of the ocean – and is therefore essential for monitoring sea-level rise. They show that sea level has been rising at an average rate of about 3 mm a year since these records began in 1993. However, recent re-analysis of these records has shown that sea-level rise is now accelerating because of global warming.